1. Field of the Invention
This invention relates to tracing buried conductors to locate conductor-to-earth leakage paths through the insulation of the buried conductors.
2. Description of the Prior Art
Telephone and electrical power lines are frequently buried in the earth for esthetic reasons and to reduce exposure to physical damage. These lines, however, do not always remain free of physical damage. The insulating material on a buried wire, for example, may be damaged during installation or as a result of earth shifting, rodent attack, or deterioration. When damage of this sort occurs, moisture in the earth often penetrates the insulation to provide a leakage path between the wire and the earth. On the other hand, when one or more wires are encased in a metallic sheath covered by insulating material, similar damage to this material often results in a leakage path between the metallic sheath and the earth. In either case, an electrical leakage path is provided between a metallic conductor and the earth which may have adverse effects on the intended function of the buried lines.
One technique for locating a fault of the above-described type applies an alternating test signal between the conductor and the earth and then uses a pair of earth probes to localize the fault by sensing the voltage gradient in the earth return path. This is explained in detail in "Locating Cable Faults," by Charles A. Maloney, pp. 380-394 in IEEE Transactions on Industry Applications, Vol. 1A-9, No. 4, July/August 1973.
With the above-mentioned technique, a phase reversal in the sensed voltage takes place as the probes traverse the fault. This is used to advantage in a prior art pulsed DC arrangement which detects the magnitude and polarity of probe detected pulses.
U.S. Pat. No. 3,299,351, issued Jan. 17, 1967, to D. L. Williams, discloses apparatus which uses to advantage the phase reversal that takes place at the fault. This apparatus adds a second signal at one-half the frequency of the test signal to the test signal before it is applied to the conductor. Both signals are sensed in the earth return path and separated from one another. The lower frequency signal is squared and then the second harmonic component is extracted and used as a reference signal. The phasal relationship between the detected test signal voltage gradient and the reference signal is indicative of the direction of the fault.